Petroleum is found in subterranean formations or reservoirs in which it has accumulated, and recovery is initially accomplished by pumping or permitting the petroleum to flow to the surface of the earth through wells drilled into and in fluid communication with the subterranean reservoirs. Petroleum can be recovered from the subterranean formations only if certain conditions exist. There must be an adequately high concentration of petroleum in the formation, and there must be adequate permeability or interconnected flow channel throughout the formation to permit the flow of fluid therethrough if sufficient pressure is applied to the fluid. When the subterranean, petroleum-containing formation has natural energy present in the form of an active, underlying or edge water drive, solution gas, or a high pressure gas cap above the petroleum within the reservoir, the natural energy is utilized to recovery petroleum in what is commonly referred to as primary recovery. In this primary phase of petroleum recovery, petroleum flows to wells drilled into and completed in the formation, the petroleum being displaced through the formation toward the wells by the naturally occurring energy in the reservoir. When the natural energy source is depleted, or in the instance of those formations which do not originally contain sufficient natural energy to permit primary recovery operations, some form of supplemental recovery process must be applied to the reservoir. Supplemental recovery is frequently referred to as secondary or tertiary recovery, although in fact it may be primary, secondary or tertiary in sequence of employment. Petroleum recovery operations involving the injection of water into the formation for the purpose of displacing petroleum toward the production well, commonly referred to as waterflooding, is the most economical and widely practiced form of supplemental recovery. Water does not displace petroleum with high efficiency, however, since water and oil are immiscible and the interfacial tension between water and oil is quite high. Persons skilled in the art of oil recovery have recognized this limitation of waterflooding and many additives have been described in the literature for incorporating in the flood water for the purpose of decreasing the interfacial tension between the injection water and the formation petroleum. For example, petroleum sulfonates have been disclosed in many references for use in oil recovery operations, but petroleum sulfonates have limitations with respect to formation water salinity, hardness and other factors which frequently restrict their usefulness. U.S. Pat. No. 3,811,504 describes a three component surfactant system comprising an anionic surfactant such as an alkyl or alkylaryl sulfonate, a nonionic surfactant such as a polyethoxylated alkylphenol, and an alkylpolyethoxy sulfate. U.S. Pat. No. 3,811,505 describes a dual surfactant system employing an anionic surfactant such as an alkyl or alkylaryl sulfonate or phosphate, plus a nonionic surfactant such as polyethoxylated alkylphenol or polyethoxylated aliphatic alcohol. These systems permit the use of a surfactant flooding process in formations containing from 500 to 12,000 parts per million polyvalent ions such as calcium and magnesium. The foregoing process employing nonionic surfactants such as ethoxylated and sulfated surfactants are effective at high salinities, but have a limited temperature tolerance range. Surfactant waterflooding processes employing alkylpolyalkoxyalkylene sulfonates or alkylarylpolyalkoxyalkylene sulfonates are described in U.S. Pat. Nos. 3,827,497; 3,890,239; and 4,018,278. These surfactant mixtures are especially suitable for use in surfactant waterflood operations being applied to formations whose temperatures exceed the useful limits of nonionic surfactants and polyethoxylated and sulfated surfactants, and are additionally effective for use in formations containing very high salinity formation water.
While the foregoing described surfactant waterflood oil recovery processes have produced various encouraging results in laboratory experiments, field application of these processes have generally been less successful than expected, and the amount of additional oil recovered has thus far been insufficient to justify the cost of the surfactant materials incorporated in the flood water. A substantial cause for the disappointing results obtained in field application of surfactant waterflood oil recovery processes described in the literature are believed to be the loss of surfactant from the aqueous surfactant fluid to the formation as the fluid passes through the flow channels of the formation. This loss of surfactant is at least in part related to the adsorption of the surface active agents from the aqueous fluid onto the mineral surface of the formation matrix. It is also believed that some loss of surfactant to the formation occurs as a consequence of surfactant fluid entering dead-end flow channels of the formation, and remaining trapped in those flow channels and unavailable for subsequent low surface tension displacement of petroleum as the fluids are displaced through the formation by the drive water.
The use of many additives has been described in the literature for the purpose of decreasing the amount of surfactant adsorbed by the formation, including water soluble salts of carbonates, phosphates, fluorides, as well as quaternary ammonium salts. Unfortunately, many of the most effective sacrificial agents cannot be employed in processes being applied to formations containing water having dissolved therein relatively high concentrations of divalent ions such as calcium and magnesium because of the precipitation of insoluble calcium or magnesium salts which occurs when the injected chemicals contact the hard formation water. In formations containing high concentrations of divalent ions, water soluble lignosulfonate salts and related compounds can be used to reduce adsorption of surfactant, but they are expensive and not entirely satisfactory for preventing loss of surfactant in the formation from the surfactant-containing fluid.
In view of the foregoing discussion, it can be appreciated that there is a significant unsatisfied commercial need for a surfactant waterflooding oil recovery method, especially one employing the synthetic surfactants which can be used in formations containing high salinity, hard water, without experiencing significant loss of surfactant from the surfactant-containing fluid to the formation mineral matrix. More particularly, there is a significant commercial need for a method of conducting a surfactant waterflooding oil recovery process in which the amount of additional oil recovered as a consequence of injecting the surfactant-containing fluid, is sufficient to justify the high cost of the surfactant waterflooding oil recovery process.